The present invention relates to a method for growing single crystals of dissociative compounds which are especially suitable for the production of substrates for both high speed and/or optical devices.
Conventionally, highly dissociative compound semiconductors, such as GaAs single crystal, are produced by the Horizontal Bridgeman method (hereinafter referred to as the HB method) or by the Liquid Encapsulated Czochralski method (hereinafter referred to as the LEC method). These conventional methods, however, have the following disadvantages and cannot sufficiently meet the following requirements. For instance, round semi-insulating (100) wafers which are free from dislocations and have a high purity and a uniform resistance are desired for use in IC substrates. However, the HB method is disadvantageous in that since an ingot with a &lt;111&gt; direction is grown in a quartz boat, the ingot must be cut into wafers at an angle of 54.7.degree. with respect to the growing direction of the crystal and then the wafers must be cut out into a round shape, when the semi-insulating wafers are requested. A further disadvantage is in that it is difficult to grow a single crystal with a high purity due to the contamination of Si from the quartz boat, as compared to the LEC method.
On the other hand, the LEC method is advantageous in that crystal growth in a &lt;100&gt; direction can be readily achieved and since B.sub.2 O.sub.3 covering the surface of GaAs melt acts as a getter for depriving Si contained in the melt, it is easy to grow single crystals with a high purity. It has been recognized that in order to obtain semi-insulating crystals, it is very important to maintain the GaAs melt in a proper composition. However, in the conventional LEC method, it is very difficult to precisely control the composition of the melt placed below the B.sub.2 O.sub.3 layer during the course of growing and an unfavorable reduction in resistance is apt to occur due to an unwanted compositional change in the melt. For this reason, it is highly desirable to develop an improved method for controlling precisely the composition of the melt throughout the growing process whilst retaining the ability of B.sub.2 O.sub.3 as a getter.
As an improvement of the LEC method, a new method and apparatus for growing crystals in an arsenic atmosphere has been proposed in Japanese patent laid-open No. 55-80796.
The new method has an advantage that it is possible to provide a wafer having a crystal defect density reduced to the same or lower level as compared with the HB method, but disadvantageously the method cannot successfully control the composition of the GaAs melt during the course of growing process, as in the case of the usual LEC method, because As cannot enter the melt through a B.sub.2 O.sub.3 layer.